Repeatability of microenvironment-specific nesting behaviour in a turtle with environmental sex determination

Evolutionary thermal adaptation is increasingly being elucidated in a varie ty of systems. However, one of the most striking examples, temperature-depe ndent sex determination (TSD) in reptiles, has proven stubbornly difficult to decipher. Theoretical models suggest that selection on and heritable var iation in thermal sensitivity of embryonic sex determination and maternal b ehavioural choice of thermal qualities of nest sites control the microevolu tionary potential of TSD. To begin addressing this important issue, we cond ucted a multiyear field study of nesting behaviour in painted turtles (Chry semys picta), which display TSD. We detected a significant field repeatabil ity for overstory vegetation cover around nests at oviposition, a trait tha t is correlated inversely with nest temperature during embryonic sex determ ination and positively with offspring sex ratio (percentage of male). Neith er clustered nesting nor individual spatial repeatability in nest site choi ce caused this striking pattern. Instead, females consistently preferred ne st environments with particular quantities of overstory vegetation cover. T hese findings confirm a crucial assumption of models concerning the microev olution and adaptive significance of TSD in reptiles. The results also indi cate that this system may be able to evolve via long-term maternal nesting behaviour in response to skewed sex ratios caused by environmental perturba tions, such as gradual long-term climate change. (C) 2001 The Association f or the Study of Animal Behaviour.